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Pension and Other Benefit Plans - Changes in Accumulated Postretirement Benefit Obligation, Plan Assets and Funded Status (Detail) - Postretirement medical plan - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Change in Projected Benefit Obligations      
Projected benefit obligation at beginning of year $ 1,213 $ 1,108  
Service cost 32 29 $ 30
Interest cost 43 46 47
Contribution by plan participants 8 8  
Actuarial gains (losses) (128) 71  
Benefits paid (62) (49)  
Projected benefit obligation at end of year 1,106 1,213 1,108
Change in Plan Assets      
Plan assets at fair value at beginning of year 1,094 952  
Actual return on plan assets (44) 143  
Company contributions 1 40  
Contributions by plan participants 8 8  
Benefits paid (62) (49)  
Plan assets at fair value at end of year 997 1,094 $ 952
Unfunded Liability (109) (119)  
Amounts Recognized in Accumulated Other Comprehensive Loss      
Actuarial losses 14 36  
Prior service credit (186) (215)  
Total amount recognized in accumulated other comprehensive income $ (172) $ (179)